1. Field of the Invention
The present invention relates to an apparatus and method for improving flue gas recirculation to minimize the oxides of nitrogen in exhaust emissions.
2. Related Information
Nitrogen oxides (xe2x80x9cNOxxe2x80x9d) are among the primary air pollutants emitted from combustion processes. NOx emissions have been identified to contribute to ground-level ozone formation, visibility degradation, acid rain and human health concerns. As a result environmental regulations have been the main driver forcing industry to install systems to control NOx emissions.
There are two primary sources for NOx generated during combustion: Fuel NOx and Thermal NOx. NOx formed due to conversion of chemically bound nitrogen is referred to as Fuel NOx. Thermal NOx refers to NOx formed from high temperature oxidation (or xe2x80x9cfixationxe2x80x9d) of atmospheric nitrogen. NO is the major constituent of thermal NOx and its formation can be modeled by the Zeldovich equation:
[NO]=k1xc2x7exp(xe2x88x92k2/T)xc2x7[N2]xc2x7[O2]xc2xdxc2x7t
where, [ ]=mole fraction, k""s=constants, T=temperature, and t=residence time. The Oxidation of Nitrogen in Combustion and Explosion, J. Zeldovich, Acta Physiochim, U.S.S.R. (Moscow), 21 (4), pp577-628 (1946). The equation indicates that NOx formation is an exponential function of temperature and a square root function of oxygen concentration. Thus, by manipulating the temperature or oxygen concentration the formation of thermal NOx can be controlled. The main control strategies for reducing thermal NOx emissions can be characterized into two types: (i) Stoichiometry-based combustion modification systems designed to control the mixing of fuel and air to modify the concentration of oxygen in the flame zone, and (ii) Dilution-based combustion modification systems designed to reduce flame temperature in the flame zone. Post Combustion control of flue gas to remove NOx such as Selective Catalytic Reduction (SCR) and Non-Selective Catalytic Reduction (NSCR) are not only expensive but also operate on a different principle from the present invention.
Stoichiometry-based Combustion Control techniques involve altering the oxygen concentration in the flame zone to lower NOx formation. Examples for stoichiometry-based combustion controls include: Low NOx Burners and Off-Stoichiometric Combustion (e.g., Over Fire Air, and Burners Out of Service). These technologies effectively control NOx emissions by providing air staging to create an initial, fuel-rich zone (partial combustion zone) followed by an air-rich zone to complete the combustion process. Some burner manufacturers also offer fuel staging, which results in ultra low levels of NOx, primarily because they are also designed to recirculate flue gas.
Dilution-based Combustion Control techniques such as Flue Gas Recirculation and Water/Steam Injection control technologies reduce thermal NOx formation by introducing inerts which absorb heat, thereby, reducing peak flame temperatures. Although dilution methods also reduce oxygen concentration in the flame zone, little reduction in NOx is expected from this mechanism. Water Injection reduces flame temperatures by absorbing the latent heat of vaporization, and as such, it results in decreasing the efficiency. Thus, it is mainly recommended as a temporary control measure to reduce NOx during peaking periods.
Flue Gas Recirculation (xe2x80x9cFGRxe2x80x9d) technology, also referred to as Windbox-FGR, does not suffer from this handicap and has minimal impact on efficiency. In a typical Windbox-FGR application, about 10 to 25% of the flue gases are recycled back to the combustion zone resulting in NOx reduction of up to 80%. Recirculating flue gas back to the combustion zone has been one of the most effective methods of reducing NOx emissions from gas and oil fired boilers since the early 1970""s. In conventional applications, the recirculated flue gas is typically extracted from the combustion unit""s outlet duct, upstream of the air heater. The flue gas is then returned through a separate duct and hot gas fan to the combustion air duct that feeds the windbox. The recirculated flue gas is mixed with the combustion air via air foils or other mixing devices in the duct. Windbox-FGR systems require installation of a separate hot gas FGR fan to move flue gas from the boiler exit to the air supply ducting at the windbox inlet, where mixing of the air and flue gas must be uniformly achieved by installation of appropriate mixing devices.
Most of the cost associated with Windbox-FGR technology is due to an additional fan or hot gas fan requirement to transport the flue gas. Thus, there is a need in the art for a simple and inexpensive flue gas recirculation system.
The present invention also reduces the cost by eliminating the need for a hot gas fan to redirect the flue gas into the combustion zone. The present invention eliminates the need for a separate hot gas fan (or FGR fan) and windbox mixing devices. U.S. Pat. No. 6,247,917 is a passive FRC which does not require a hot gas fan and obtains a pressure drop in the recirculation line by adjusting the size of the air intake line. The present invention does not require an air intake line, but is fully operably if one is present, without any sizing nor does the present invention utilize a hot gas fan.
Briefly the present invention is an apparatus and process for improved flue gas recirculation wherein the apparatus comprises: a combustion unit, an exhaust duct for removing flue gas from the combustion unit, an air fan having an inlet for delivering combustion air through said inlet into said combustion unit for combustion of a fuel therein, and a recirculation line penetrating into the exhaust duct and having an inlet therein for capturing and directing a portion of said flue gas, said recirculation line being connected to said air fan inlet to provide induction of flue gas by said air fan into said combustion unit. The portion of the recirculation line that extends into the exhaust duct is preferably aerodynamically configured to capture a portion of a flue gas stream without detrimental impedance of the gas flow. Various configurations may be used such as a closed tubular member having an opening facing toward the flue gas flow, which serves to capture a portion of the gas and facilitate its recycle by using some of the dynamic energy in the gas flow.
The recirculation line may be connected directly to cover a portion of the inlet of the air intake fan or into an air line attached to said inlet. The term xe2x80x9ccombustion unitxe2x80x9d includes fired heaters, boilers, ethylene furnaces, incinerators, steam generators, process heaters and the like. The term xe2x80x9cductxe2x80x9d as used herein includes ducts, lines, flues, stacks and any equivalent elements.